Conventionally, for example, as disclosed in International Publication No. WO 2014/115272, a method for detecting a desaturation voltage of a semiconductor switching device is known as a short-circuit protection function of a semiconductor switching device. In general, when the semiconductor switching device is in an ON state, a voltage between terminals of the semiconductor switching device decreases and is stabilized at a minimum voltage. This minimum voltage is also referred to as a “saturation voltage”. Specific examples of the “voltage between terminals” described herein include a collector-emitter voltage when the semiconductor switching device is, for example, an insulating gate bipolar transistor (IGBT), and a source-drain voltage when the semiconductor switching device is a MOSFET. When the semiconductor switching device is in a short-circuit state, the voltage between terminals of the semiconductor switching device increases from the saturation voltage due to overcurrent. When the voltage between terminals increases from the saturation voltage, the voltage becomes an abnormal voltage at which the voltage between terminals does not match the saturation voltage, i.e., a desaturation voltage. A short-circuit of the semiconductor switching device can be detected by detecting this desaturation voltage.
In a technique disclosed in International Publication No. WO 2014/115272, a high withstand diode and a high withstand capacitive element are provided between a semiconductor device driving circuit and a semiconductor switching device, and the capacitive element is charged when the semiconductor switching device is in a desaturation state, to detect that the semiconductor switching device is in the desaturation state. Specifically, when the capacitive element is charged by a constant current circuit, a voltage at a terminal VDSH increases. When the voltage at the terminal VDSH has reached a certain value or higher, it is determined that a desaturation voltage is generated in the semiconductor switching device.